David Horowitz, founder of Students for Academic Freedom, addresses a public hearing in 2006.
Photograph: Horry Rumph Jr/AP

David Horowitz, a 79-year-old former Black Panther party activist turned conservative firebrand, was feeling defensive on behalf of his friend and protege Stephen Miller, the 32-year-old architect of Donald Trump’s immigration policy.

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“He’s a very smart young man,” Horowitz said in an expletive-laden phone interview with the Guardian. “Here’s the issue: should America, like every other fucking country in the world, particularly Mexico, have borders? That’s the issue. And the Democrats have just demagogued it to make it anti-immigrant. It’s bullshit.”

Horowitz’s tone captured the wrath and upset that spilled across the country as Americans learned new details of a Trump administration program to separate migrant children from their parents at the southern border.

Late on Tuesday, the Associated Press revealed that the government had established at least three “tender age” shelters in south Texas for babies and other young children forcibly separated from their parents. More than 2,300 children have been separated from 2,206 adults with whom they traveled, according to Department of Homeland Security figures.

Before Trump said on Wednesday he would end the policy, a barrage of such alarming scenes focused public attention on the most consistent White House voices in favor of family separation – Miller and the attorney general, Jeff Sessions, both longtime immigration hardliners.

“The message is that no one is exempt from immigration law,” Miller told the New York Times last week.

The government’s hands are tied by Obama and the Democrats, so if a kid gets in, everybody gets in

David Horowitz

But Sessions and Miller share more than policy views: they share a mutual friend in Horowitz, who first met Miller when the younger man was still in high school, spoke at Duke University at Miller’s invitation in 2006, and who ultimately put Miller forward for a job in Sessions’s office when Sessions was a senator from Alabama.

Miller, Sessions and another key figure in Trump’s orbit, former Breitbart chief Steve Bannon, were together at a 2014 awards ceremony in Florida hosted by Horowitz, who separately conferred an award on Bannon after he left his role as chief White House strategist.

The intersecting paths connecting Trump-world figures to Horowitz, once a leading intellectual of the New Left who studied in England under the Marxist sociologist Ralph Miliband before a midlife political conversion, would not seem to bear the same significance for policy as the president’s own leadership.

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But the astringency of Horowitz’s rhetoric on immigration and other issues, which has won him censure for extremist speech by the Southern Poverty Law Center, is readily recognizable from the podium in the White House briefing room, on television on the lips of Trump proxies such as the former campaign manager Corey Lewandowski, and in the policy of family separation at the southern border.

“It’s not a policy of family separation,” said Horowitz. “It’s the government’s hands are tied by Obama and the Democrats, so if a kid gets in, everybody gets in. It’s disgusting. It’s an abusive exploitation of these kids.”

‘There’s a fucking lynching going on’

While Horowitz’s most incendiary rhetoric these days aims at America’s racial and gender divides – he has said that women “have different aptitudes than men in math”, called the author Ta-Nehisi Coates “just a rank racist” and attacked African Americans killed by police as “criminals who prey on black people” – he is quick to lay down fire on the immigration issue, and to defend Miller.

Stephen Miller is seen in the background, center, as Donald Trump and the House majority leader, Kevin McCarthy, leave a meeting on Capitol Hill on Tuesday. Photograph: Pablo Martinez Monsivais/AP

“There’s a fucking lynching going on here, and there’s a wolfpack that your newspaper is part of,” Horowitz said. “It’s gross. The kid was a liberal. His family are Santa Monica liberals. And I met him, he was like 17 or 18.”

When Horowitz, a Los Angeles resident, first noticed Miller, the precocious teen was lobbying his left-leaning high school to host daily recitations of the pledge of allegiance in the wake of the 9/11 attacks.

“Even then I was impressed by how articulate and smart this young man was,” Horowitz later wrote, “and that he pulled no punches, so unusual in the conservative circles I was familiar with.”

Of his efforts to get Miller a job on Capitol Hill after Miller graduated from Duke, he said: “I recommended Steve to Sessions. Steve’s a brilliant guy, he’s been working on immigration policy, it’s very clear that his view of immigration is exactly what it has been in this country for 200 years until Ted Kennedy revised all the fuckin’ laws. The Democrats did this. You can’t have a country with no borders. Trump is 100% right.”

Few share Horowitz’s view of Miller as a historic centrist on immigration, however, including Republicans on Capitol Hill. “As long as Stephen Miller’s in charge of negotiating immigration we’re going nowhere,” the Republican senator Lindsey Graham told an interviewer in January. “He’s been an outlier for years.”

Steve’s a brilliant guy … his view of immigration is exactly what it has been in this country for 200 years

David Horowitz

Raised by card-carrying communists in the Queens borough of New York City, Horowitz was a founder of the New Left movement, editing Ramparts magazine and later raising money for the Black Panther party.

But after the 1975 murder of an acquaintance that he believed was carried out by “my Black Panther comrades”, he wrote in a 1986 essay, Horowitz grew disillusioned with the Left, which he saw as shielding the killers.

Following a series of biographies of American family dynasties, he published a memoir of his political conversion, Radical Son, and more than a dozen blistering political tracts culminating in the 2017 volume Big Agenda: President Trump’s Plan to Save America.

Horowitz says he is drawn to ideological counterparts who have what he sees as a warrior instinct akin to that of his former “Black Panther comrades”. Miller and Bannon have it, Horowitz said.

The men also share a vision of a country threatened by the malign forces of unchecked immigration and “radical Islamic terror”, and weakened on the inside by what they diagnose as decades of moral rot from “political correctness” and progressive politics embodied by Barack Obama.

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After Horowitz promoted Miller’s campaign against his high school, Miller founded chapters of one of Horowitz’s activist groups, Students for Academic Freedom, at school and at Duke, from which Miller graduated in 2007.

Miller hosted Horowitz for a speech at Duke in 2006, at which Miller, clearly relishing the tension of an auditorium filled with protesters, introduced Horowitz as “one of the most influential political thinkers in America today”.

Of Bannon, Horowitz says: “I like that he was a fighter.” In 2017, a Horowitz group honored Bannon with an award for bravery named after the first person to go over Niagara Falls in a barrel and survive. Bannon featured Horowitz as a talking head in a 2012 film that attacked the Occupy movement.

Speaking to the Guardian, Horowitz showed he can still fight, defending a Trump tweet saying that Democrats “don’t care about crime and want illegal immigrants, no matter how bad they may be, to pour into and infest our Country, like MS-13”.

“That quote has nothing to do with immigrants,” Horowitz said. “It has to do with illegal. And that little distinction is a huge difference.”